1. Field of the Invention
The invention is related to the field of magnetic disk drive systems and, in particular, to patterned magnetic media having an exchange bridge structure connecting the islands of a perpendicular magnetic recording layer.
2. Statement of the Problem
Many computer systems use magnetic disk drives for mass storage of information. Magnetic disk drives typically include one or more magnetic recording heads (sometimes referred to as sliders) that include read elements and write elements. An actuator/suspension arm holds the recording head above a magnetic disk. When the magnetic disk rotates, an air flow generated by the rotation of the magnetic disk causes an air bearing surface (ABS) side of the recording head to ride a particular height above the magnetic disk. The height depends on the shape of the ABS. As the recording head rides on the air bearing, an actuator moves the actuator/suspension arm to position the read element and the write element over selected tracks of the magnetic disk.
One type of magnetic disk presently used in magnetic disk drives provides for longitudinal recording. A magnetic disk for longitudinal recording includes a magnetic recording layer having an easy axis of magnetization parallel to the substrate. The easy axis of magnetization is the crystalline axis that is aligned along the lowest energy direction for the magnetic moment. Another type of magnetic disk provides for perpendicular recording. A magnetic disk for perpendicular recording includes a magnetic recording layer having an easy axis of magnetization oriented substantially perpendicular to the substrate.
A perpendicular magnetic disk is generally formed with a soft magnetic underlayer (SUL), an interlayer, a perpendicular magnetic recording layer, and a protective layer or overcoat for protecting the surface of the perpendicular magnetic recording layer, which are formed on a substrate. The soft magnetic underlayer (SUL) serves to concentrate a magnetic flux emitted from a main pole of a write element and to serve as a flux return path back to a return pole of the write element during recording on the magnetic recording layer. The interlayer serves to control the size of magnetic crystal grains and the orientation of the magnetic crystal grains in the magnetic recording layer. The interlayer also serves to magnetically de-couple the SUL and the magnetic recording layer.
On a longitudinal or perpendicular magnetic disk, the magnetic recording layer is divided into small magnetic regions, each of which is used to encode a single binary unit of information. The magnetic regions include multiple magnetic grains, possibly small in number (10 to 100), which generates a highly localized magnetic field. The write element magnetizes a magnetic region by generating a strong local magnetic field.
As the areal density of magnetic disks increase, the super-paramagnetic effect causes problems for disk manufacturers. The super-paramagnetic effect occurs when the microscopic magnetic grains on the disk become so tiny that ambient temperature can reverse their magnetic orientations. The result is that the bit is erased and the data is lost.
One solution to the problems posed by the super-paramagnetic effect is to pattern the magnetic disk (also referred to as bit patterned recording). A patterned disk is created as an ordered array of highly uniform islands, with each island capable of storing an individual bit. A patterned disk may allow ultra-high density recording to be achieved. Because each island represents an individual magnetic domain, the patterned disk is thermally stable and higher densities may be achieved.
However, the magnetic stability of the islands as well as their switching field distribution may be adversely affected by magnetostatic interaction fields. The adverse effects of magnetostatic interaction fields are a problem especially when the media is patterned with high densities of islands. As the size of the islands decrease with increased density, the width of the write element (i.e., pole tip) also decreases. Smaller pole tips lead to reduced write fields. Thus, the switching field (Hs) of the islands needs to be decreased, which leads to decreased energy barriers against thermal reversal or demagnetization-induced reversal.
The islands on a perpendicular magnetic disk are patterned as discrete magnetic islands, meaning that there is no magnetic material connecting the islands. Although there is no magnetic material connecting the islands, there is still magnetostatic coupling between the islands. Magnetostatic coupling tends to cause antiparallel (AP) coupling between neighboring islands. As the densities increase and the islands are patterned closer together, the magnetostatic coupling between the islands increases. The increase in magnetostatic coupling can de-stabilize the island magnetization.